Title

Author

Degree Type

Dissertation

Date of Award

2009

Degree Name

Doctor of Philosophy

Department

Chemical and Biological Engineering

First Advisor

Brent H. Shanks

Abstract

Potassium-promoted iron oxide is the primary catalyst for dehydrogenating ethylbenzene to styrene. Due to an increasing demand for saving energy, there is a strong incentive to operate the reaction at reduced steam/ethylbenzene molar ratio, since a large amount of steam is used in the process. However, the catalyst experiences short-term deactivation under low S/EB conditions. Active site blocking by surface carbon and iron oxide reduction by either surface carbon or H2 are two possible deactivation mechanisms. However, the relative importance of these two mechanisms is not understood.

It is very important to understand which deactivation mechanism dominates as different mechanism will lead to different development approaches. In this study, phase transitions of iron oxide based catalyst samples were investigated with TGA and XRD to understand the intrinsic deactivation mechanism. The effects of various promoters on iron oxide activity and stability were also studied. Hydrogen and carbon dioxide were utilized as the gas environment individually to avoid convolution of effects. Ethylbenzene was then applied to characterize the combined effects of hydrogen, carbon dioxide, and surface coke.

Potassium efficiently increases the activity of iron oxide and its effect on phase stability was examined. The active potassium ferrite phase and potassium polyferrite, which has been considered a storage phase of potassium and iron (III), can be converted to each other when exposed to carbon dioxide or hydrogen. It was also found that the deposited surface carbon was a stronger reductant than hydrogen.

Other minor promoters are also used in dehydrogenation catalysts to enhance stability, enhance activity, or increase the styrene selectivity. Therefore, their effects on the catalyst were also examined in this study. Chromium, calcium, and cerium were found to have a positive effect on iron oxide stability, while vanadium and molybdenum had negative impacts on iron oxide stability. Activity enhancement could be achieved by doping with chromium, calcium, molybdenum, and cerium. Vanadium greatly reduced the activity of catalyst, since it inhibited formation of the active phase.